(1) Field of the Invention
The present invention relates to a control valve for a variable displacement compressor, and more particularly to a control valve for a variable displacement compressor which is suitable for controlling discharging capacity of refrigerant of a variable displacement compressor for an automotive air conditioner.
(2) Description of the Related Art
A compressor used in a refrigeration cycle of an automotive air conditioner is driven by an engine whose rotational speed varies depending on a traveling condition of the vehicle, and hence incapable of performing rotational speed control. To eliminate the inconvenience, a variable displacement compressor capable of changing the discharge amount of refrigerant is generally employed so as to obtain an adequate refrigerating capacity without being constrained by the rotational speed of the engine.
In a typical variable displacement compressor, a wobble plate is disposed within a crankcase formed gastight, such that the inclination angle thereof can be changed, and driven by the rotational motion of a rotational shaft, for performing wobbling motion, and pistons caused to perform reciprocating motion in a direction parallel to the rotational shaft by the wobbling motion of the wobble plate draw refrigerant from a suction chamber into associated cylinders, compress the refrigerant, and then discharge the same into a discharge chamber. In doing this, the inclination angle of the wobble plate can be varied by changing the pressure in the crankcase, whereby the stroke of the pistons is changed for changing the discharge amount of the refrigerant. The control valve for a variable displacement compressor provides control to change the pressure in the crankcase.
In general, the control valve for variably controlling the displacement of the compressor introduces part of refrigerant discharged at discharge pressure Pd from the discharge chamber into the crankcase formed gastight, and controls pressure Pc in the crankcase through control of the amount of refrigerant thus introduced. The amount of introduced refrigerant is controlled according to suction pressure Ps in the suction chamber. That is, the control valve for a variable displacement compressor senses the suction pressure Ps, and controls the flow rate of refrigerant introduced at discharge pressure Pd from the discharge chamber into the crankcase, so as to maintain the suction pressure Ps at a constant level.
To this end, the control valve for a variable displacement compressor is equipped with a pressure-sensing section for sensing the suction pressure Ps, and a valve section for causing a passage leading from the discharge chamber to the crankcase to open and close according to the suction pressure Ps sensed by the pressure-sensing section. Further, a type of the control valve for a variable displacement compressor which is capable of freely externally setting a value of suction pressure Ps to be assumed at the start of the variable displacement operation is equipped with a solenoid that enables configuration of settings of the pressure-sensing section by external electric current.
By the way, conventional control valves for variable displacement compressors which can be externally controlled include a type for controlling a so-called clutchless variable displacement compressor that is configured such that an engine is directly connected to a rotational shaft on which a wobble plate is fitted, without providing a solenoid clutch between the engine and the rotational shaft for execution and inhibition of transmission of a driving force of the engine (e.g. Japanese Unexamined Patent Publication (Kokai) No. H06-346845).
This control valve comprises a valve section causing a passage communicating between a discharge chamber and a crankcase to be opened and closed, a bellows as a pressure-sensing section, which is integrally connected to the valve element of the valve section, for causing the valve section to operate in the opening direction as the suction pressure Ps becomes lower, and a solenoid for generating an electromagnetic force causing the valve section to operate in the closing direction with the bellows being fixedly attracted to a movable core thereof, the valve section, the bellows, and the solenoid being arranged in this order. Therefore, when the solenoid is not energized, the valve section is basically fully open, whereby pressure Pc in the crankcase can be maintained at a level close to the discharge pressure Pd. This causes the wobble plate to become substantially at right angles to the rotational shaft, enabling the variable displacement compressor to operate with the minimum capacity. Thus, the discharging capacity of refrigerant can be substantially reduced to approximately zero even though the engine is directly connected to the rotational shaft, whereby the solenoid clutch can be dispensed with.
However, the above-described conventional control valve for a variable displacement compressor suffers from the problem that due to direct transmission of the expanding and contracting motion of the bellows to the valve element, the valve element is moved in the valve-opening direction by contraction of the bellows, particularly in the case where due to a heavy refrigeration load, the suction pressure Ps is high, e.g. when the outside temperature is high. This prevents the valve section from being fully opened even though the solenoid is not energized.